I think I found a bug with the function getBookableRoutes
According to the wiki entry about the function it returns several codes about the success of the operation ( SUCCESS, NOT FOUND, FLIGHT BOOKED, GROUNDED, SUSPENDED, VA INACTIVE ).
But if I try to book a flight when there is already one booked, the function does not return an appropriate code (e.g. FLIGHT BOOKED).
Instead it produces an error message

Warning: DOMDocument::load() [domdocument.load]: XML declaration allowed only at the start of the document in http://www.fsairlines.net/va_interface.php5?va_id=XXXX&function=getBookableRoutes&pilot_id=XXXXX